Fertilising a planted aquarium — beginner guide

"Do I need to fertilise a planted tank?" If you have plants — yes. Even in low-tech, even with fish producing organics. Fertilisation isn't "pro-only" — it's a standard piece of husbandry, just like water changes.

This guide walks you through the basics: what plants need, the three most popular dosing schools, what to buy first, how to spot deficiencies and excesses, and a practical schedule for a beginner with a planted tank.

1. What plants need

Aquarium plants need the same things as land plants: macronutrients, micronutrients, and a carbon source (CO₂).

Macros — the "fuel"

• Nitrogen (N) — as NO₃⁻ (nitrate). Chlorophyll component. Produced by the nitrogen cycle.
• Phosphorus (P) — as PO₄ (phosphate). Needed for DNA, cell membranes.
• Potassium (K) — osmotic regulation, enzymes. Not produced by fish — has to be dosed.
• Magnesium (Mg), Calcium (Ca) — in hard water (GH). In soft water needs supplementing.

Micros — the "vitamins"

• Iron (Fe) — the key micro. Chlorophyll, oxygen transport in the plant. First to run out.
• Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo) — trace amounts, usually in a single "micro mix".

CO₂ — the carbon source

Plants build ~45% of their dry mass from CO₂. In a tank without injection, CO₂ availability is the growth limiter (~3–5 mg/L from surface exchange). With injection: 25–35 mg/L for 8–10 h per day.

Where each comes from

2. Three dosing schools

1. All-in-one (for beginners)

Tropica Premium Nutrition, Easy Life Profito, Seachem Flourish Comprehensive. One bottle contains all macros and micros in predictable ratios.

• How to dose: once a week per instructions. Typically 1–2 ml per 50 L.
• Pros: simple, hard to overdose, doesn't require testing.
• Cons: more expensive long-term, fixed ratios (harder to fine-tune).
• For: low-tech, first planted tank, don't feel like mixing.

2. Estimative Index (EI) — for the ambitious

Tom Barr's method. You dose your own salts (KNO₃, KH₂PO₄, K₂SO₄, micros) in excess so plants have everything on tap. A 50% weekly water change resets the accumulating macros.

• How to dose: macros 2–3 times a week, micros 2–3 times (different days). 50% water change weekly.
• Pros: great results in high-tech, much cheaper, faster growth.
• Cons: needs CO₂ to fully utilise, more water-change work, need to buy salts.
• For: high-tech with CO₂, experienced aquarists.

3. PPS-Pro — "precise" low-tech

Perpetual Preservation System. You dose small amounts daily, matched to actual plant uptake. Fewer water changes.

• How to dose: small daily doses (0.5–2 ml).
• Pros: stable parameters, fewer water changes (20% every 2 weeks), cost-effective.
• Cons: needs regular testing (NO₃, PO₄), harder to start.
• For: low-tech, people with time to dose daily but not for big water changes.

3. What to buy first

Minimum kit (low-tech, first time)

• All-in-one fertiliser (Tropica Premium Nutrition 300 ml ~$12 — lasts 3–4 months).
• Liquid tests: NO₃, PO₄, GH (for monitoring).
• Syringe 5 ml for precise dosing.

Starter cost: ~$25. Enough to run a tank with Anubias, Microsorum, Cryptocoryne, Vallisneria for a year.

For the ambitious (EI / PPS-Pro)

• KNO₃ (potassium nitrate, N + K) — 1 kg ~$8, lasts ~2 years.
• KH₂PO₄ (monopotassium phosphate, P + K) — 500 g ~$10, lasts 3+ years.
• K₂SO₄ (potassium sulphate, K only) — 1 kg ~$7.
• Micro mix (Tropica Premium Specialised, Seachem Flourish, Easy Life Ferro) — contains Fe and other micros.
• MgSO₄ (Epsom salt) — 500 g ~$4, if you have soft water.
• Syringes 5 ml and 20 ml, jeweller's scale (0.01 g, ~$20).

Starter cost: ~$60. Lasts 2–3 years of dosing.

Active substrate + root tabs

• Active substrate (ADA Amazonia, Tropica Soil, JBL AquaBasis) — invest at setup. Feeds for 2–3 years.
• Root tabs (JBL The 7 Balls, Tropica Plant Nutrition Capsules) — push into substrate next to root-feeders (Echinodorus, Cryptocoryne) every 2–4 months.

4. Deficiency symptoms — quick diagnosis

Healthy plants have uniform green colour and steady growth. Problems show up on either old or new leaves — depending on the element.

Rule: old leaves yellow → mobile elements (N, P, K, Mg — plant reallocates to new leaves). Young leaves suffer → immobile (Fe, Ca, boron — plant can't move them).

5. Symptoms of excess

Overfertilising is less common than deficiencies but looks ugly:

• Black brush algae (BBA), green dust, hair algae — often from PO₄ or micro overdose. See the algae guide.
• Brown coating on leaves — iron overdose. Lower the micro dose.
• Water stained "dirty" — excess micros, especially Fe. Activated carbon in the filter for a week.
• Apathetic fish, dead shrimp — possible copper from some micro mixes (sensitive inverts). Check the micro label.
• Dropping pH — KH₂PO₄ mildly lowers pH. Normal with large EI doses.

6. Active substrate and root tabs

Active substrate (soil)

ADA Amazonia, Tropica Soil, JBL Proflora — special substrates with "built-in" nutrients. Release a lot of ammonia at setup (longer cycling), but feed root plants for 2–3 years.

• Pros: great start for plants, fast growth, lower dosing in the first year.
• Cons: pricier (~$50–100 for a 60 L), lowers pH (not for Malawi cichlids), more frequent water changes at startup, needs replacing after 2–3 years (full reset).
• For: advanced planted tank, Amazon fish, Caridina shrimp.

Root tabs

JBL The 7 Balls, Tropica Plant Nutrition Capsules, Seachem Flourish Tabs. Nutrient capsules pushed into substrate next to plants.

• Pros: loaded with Fe, cheap (~$15 for 100 tabs), root plants (Echinodorus, Cryptocoryne, Vallisneria) grow 2× faster.
• How to use: push 1 capsule next to each larger root plant, every 2–3 months.
• Doesn't help: Anubias, Microsorum, mosses (attached, not rooted).

7. A practical schedule for beginners

100 L tank, low-tech, standard stocking & plants

1. Weeks 1–2 (after cycling): don't dose. Plants adapt, draw from substrate.
2. Week 3+: once a week, 3–4 ml Tropica Premium Nutrition (or equivalent) after a water change.
3. Every 2 months: push 3–5 root tabs next to Echinodorus, Cryptocoryne, Vallisneria.
4. Monitor: plants grow, slowly. Leaves without yellowing or holes. Algae don't dominate.

If after 6–8 weeks plants seem "stalled" or deficiency symptoms show up — bump the dose by 50% and watch.

When to switch to DIY (own salts)

Consider after 6–12 months if:

• All-in-one cost starts hurting (tank > 200 L).
• Plants struggle despite full dose (you need to tune ratios).
• Thinking about CO₂ + EI (more control = better results).

8. FAQ

Do I need to dose if I only have Anubias and Java fern?

Yes, but minimally. 1–2 ml of all-in-one every 2 weeks keeps plants from weakening. Fully undosed, Anubias survives but grows one leaf every 2 months instead of every month.

Is NO₃ from fertiliser dangerous to fish?

Not at plant-level concentrations (10–25 mg/L). Fish tolerate NO₃ up to 40 mg/L without issues. Chronic stress only shows above 50–60 mg/L.

All-in-one or own salts — which is better?

For the first year — all-in-one. Less effort, harder to mess up. After a year, if you want more control or savings — go for salts. The fertiliser calculator helps.

How long can a tank go without dosing?

A densely planted low-tech with good active substrate — a few months, plants draw from reserves. Without active substrate and with growing plants — 3–4 weeks before you see deficiencies (especially iron).

I dose but plants still yellow

Three most common causes: (1) under-dosed (bump 50%), (2) very hard water blocks Fe (try RO mix), (3) no CO₂ — plants can't use fertiliser without carbon. Check NO₃ and PO₄ are in target ranges.

Fertiliser for shrimp — watch copper?

Yes. Copper (Cu) in some micro mixes is toxic to shrimp above 0.03 mg/L. Look for "shrimp safe" or "copper-free" fertilisers (e.g. Tropica Premium Specialised is safe).

What's the role of water changes in fertilising?

Water changes "reset" accumulated substances — excess fertiliser, nitrate, organics. In EI it's a core element. With all-in-one (Tropica) — a 20–30% weekly change is plenty. See the water change guide.

When to dose — morning, evening, after feeding?

1–2 hours before lights on. Plants only start photosynthesis after a few hours of light, so the fertiliser is "ready to use" when they spin up. Remember: dose macros and micros on different days (EI) so they don't react in the water.

Wrap-up

For a beginner, fertilising is a simple routine, not a chemistry PhD:

1. Buy a bottle of all-in-one (Tropica, Easy Life, Seachem — $12 for half a year).
2. Dose once a week after a water change, per label.
3. Test NO₃ every 2–4 weeks (target: 10–25 mg/L).
4. Every 2–3 months push root tabs next to substrate-rooted plants.
5. Watch the plants — yellowing, holes, paling are signals to respond.

Once the tank stabilises and you love the topic — consider your own salts. Everything on the calculator: fertiliser calculator.

Related: easy plants for beginners, photoperiod (light + nutrients go together), water parameters (GH blocks iron).